The evolution of the refining industry has lead to the hydrotreatment of heavy hydrocarbons of petroleum acquiring a technological and economic importance similar to the processes of hydrocracking and catalytic reforming. Among the heavy hydrocarbons of petroleum are the heavy crudes, the extra-heavy crudes, blends of heavy and light crudes and petroleum residuals, such as residues from the atmospheric or vacuum distillation, which present a specific gravity less than 32° API and a content of distillates recovered @ 538° C. less than 80% by volume.
The production of heavy crudes and the availability of petroleum residuals with high sulfur and metal contents, as well as the demands regarding the improvement of fuels in their ecological quality, brought about the development and expansion of hydrotreatment processes of this type of feedstocks for their refining.
Since the 70's, there have been reports of the hydrotreatment of petroleum residuals where they state the main aim as the recovery of valuable distillable fractions with low heavy heteroatom concentration.
The heavy crudes require a treatment similar to the petroleum residuals for their processing, due to the fact that they are characterized by their low Hydrogen/Carbon (H/C) ratio, high viscosity, high content of contaminants, essentially sulfur, nitrogen and metals, and low yield of distillates.
The reactive system is the part of the process where most attention has been placed for the treatment of this type of feedstocks, which may be fixed-bed, ebullated-bed or in dispersed phase. The refining industry mostly uses the fixed-bed type.
The high concentration of metals in heavy crudes and in petroleum residuals is reflected by a fast deactivation of the catalysts, therefore it is important that these feedstocks be demetallized in a first stage of their treatment to maximize the removal of other contaminants in later stages, thereby increasing the lifecycle of the catalysts used in these stages.
The most efficient refining processes in the removal of contaminants are those of hydrotreatment, which are applied to practically all fractions of oil such as: naphthas, middle distillates, vacuum distillates, residues, etc. In the case of heavy crudes and petroleum residuals, where the desire is to simultaneously remove various contaminants, principally: metals, sulfur, nitrogen and asphaltenes; it requires an appropriate selection of the type of reactor, catalysts with high activity and selectivity for these reactions, as well as operating conditions that will render the process profitable.
The improvement of a heavy hydrocarbon of petroleum implies its processing to remove contaminants and increase the Hydrogen/Carbon (H/C) ratio, usually by the use of process schemes based on hydrotreatment.
The commercial processes currently in existence perform the hydrotreatment of heavy hydrocarbons of petroleum under operating conditions with high pressures, in the range of 140 to 220 kg/cm2 for fixed bed and ebullated-bed, which obtain high conversions. To maintain continuity in the operation of these processes, the formation of sediments and sludge is limited to a maximum content of 0.80% by weight.
The operation of the hydrotreatment processes of heavy hydrocarbons of petroleum at low pressures, less than 140 kg/cm2, has been limited by the formation of sediments and sludge, which is a characteristic problem of these processes. The formation of sediments and sludge increases when the conversion of heavy fractions (boiling point>538° C.) to light fractions also increases or by reducing the pressure in the reactors. For this reason, the commercial processes of hydrocracking of heavy hydrocarbons operate under operating conditions with high pressures, above 140 kg/cm2, in order to obtain attractive conversions of the heavy fractions.
As references of patents related with hydrotreatment processes of heavy hydrocarbons of petroleum, there are the following inventions:
The American patent U.S. Pat. No. 5,591,325 of Jan. 7, 1997, claims a catalytic process for hydrotreating heavy oils of petroleum in two stages. The first stage is carried out in a fixed bed reactor for a removal of no more than 80% of Nickel+Vanadium (Ni+V), preferably from 30 to 70%, although in the examples it states removals of between 45.3 and 47%. The operating conditions in this stage are as follows: temperature of between 320 and 410° C., pressure from 50 to 250 kg/cm2, space velocity (LHSV) of 0.1 to 2.0 h−1 and Hydrogen/Hydrocarbon (H2/HC) ratio of 300 to 1,200 nl/l. The second stage is for the removal of sulfur, nitrogen and remaining metals in an ebullated-bed reactor in the following operating conditions: temperature of 350 to 450° C., pressure 50 to 250 kg/cm2, LHSV of 0.2 to 10.0 h−1 and H2/HC ration of 500 to 3,000 nl/l.
In that regard, it is important to note that said patent precisely exemplifies hydrotreatment in two stages of reaction of an atmospheric residue in the following operating conditions: pressure of 150 kg/cm2, LHSV of 0.2 h−1, temperature of 370 and 395° C. for the first and second stages, respectively, and H2/HC ratio of 700 nl/l, thereby obtaining total removals of Ni+V of 109 wppm, total nitrogen of 1,970 wppm, insolubles in n-C7 (asphaltenes) of 6.6% by weight and total sulfur of 3.78% by weight, as well as a formation of sediments and sludge of 0.01% by weight. Said patent also claims the utilization of a catalyst based on a metal of the VIA, VIII and V groups for stage I and a catalyst with a hydrogenation metal supported in an organic oxide for stage II.
The American patent U.S. Pat. No. 5,779,992 of Jul. 14, 1998, which is in part a continuation of the American patent U.S. Pat. No. 5,591,325, relates to an apparatus which comprises: a′) a fixed-bed reactor packed with a catalyst to hydrodemetallize a heavy oil of petroleum, and b′) a suspended-bed reactor packed with a hydrodesulfurizing catalyst to hydrotreat the effluent product of the reactor of section a′). According to the apparatus of this invention, first a heavy oil of petroleum is fed into a fixed-bed reactor packed with a hydrodemetallization catalyst and then b) the heavy oil of petroleum hydrodemetallized in stage a) is fed to a suspended-bed reactor with a hydrodesulfurization catalyst in order to perform a deeper hydrotreatment thereof. The hydrotreatment may be carried out for a prolonged period of time. The operating conditions are similar to those described in U.S. Pat. No. 5,591,325.
The Mexican patent MX 179,301 of Aug. 25, 1995, granted to Instituto Mexicano del Petróleo [the Mexican Institute of Petroleum], provides a procedure for hydrotreating heavy crude oils to obtain synthetic crude, with an API gravity of gravity of 25 to 40. This process comprises the steps of: catalytic hydrotreatment of heavy crude oils with API gravity less than 24, with a final boiling temperature range from room temperature to 800° C. at a pressure of 760 mmHg and contaminant contents greater than 2% by weight of sulfur, 1,000 wppm of nitrogen, 150 wppm of metals (nickel and vanadium) and 5% by weight of asphaltenes; separation of the effluent from the reactor in a liquid phase and another phase of vapor, and carriage of the liquid phase to a scrubber. This process recovers a treated or improved crude with low contaminant content, being able to process as one feedstock 100% in a conventional refining scheme, increasing the yield of distillates and the quality thereof.
The U.S. Pat. No. 3,901,792 of Aug. 26, 1975 claims a method for demetallizing and desulfurizing crude or atmospheric residual in multiple stages. Initially, the heavy feedstock is introduced with hydrogen within an ebullated catalytic bed in the following operating conditions: pressure of 68 to 170 kg/cm2, temperature of 387 to 440° C., LHSV of 0.20 to 1.5 h−1, where the degree of demetallization is in the region of 50 to 80% by weight or more, depending on the quantity of nickel and vanadium of the feed. The light fraction leaves by the upper part of the reactor as acid gas for subsequent recovery of the light fractions of hydrocarbons, whereas the liquid effluent is conducted to a second stage of reaction mixed with a stream of hydrogen for its hydrodesulfurization in a bed of the same characteristics as that of the first stage. In the upper part of the reactor, the gaseous fraction is recovered for subsequent treatment thereof, whereas the liquid effluent recovered in this second reactor is conducted to a subsequent fractioning or treatment.
American patent U.S. Pat. No. 4,166,026 of Aug. 28, 1979, protects a two-stage process of hydrotreatment of heavy hydrocarbons, such as heavy crudes, topped crudes, vacuum residues or bituminous oils with a high content of asphaltenes, heavy metals and sulfur. The heavy oil is heated together with a stream of hydrogen for the first stage of hydrodemetallization and hydrocracking of the asphaltenes. The effluent after being subjected to this first stage, is conducted to a gas-liquid separator, where the gaseous fraction rich hydrogen, hydrogen sulfide and light hydrocarbons is conducted to a scrubber for the recovery of the light hydrocarbons, whereas the liquid effluent together with a part of the recirculating hydrogen passes to a second stage of reaction where the principal reactions of hydrodesulfurization and hydrodenitrogenation are effected. Subsequently, the effluent from this step is conducted to a gas-liquid separator, where the liquid product is recovered and conducted to separator to obtain a light fraction and a heavy fraction. Meanwhile, the gaseous fraction rich in hydrogen, hydrogen sulfide and light hydrocarbons is conducted to a scrubber for the recovery of the light hydrocarbons and the gaseous fraction rich in hydrogen and hydrogen for scrubbing in subsequent unit. The operating conditions in which the process operates preferably in both stages are as follows: pressure of 30 to 250 kg/cm2, temperature of 350 to 450° C., H2/HC ration of 100 to 2,000 normal liters per liter of charge and LHSV of 0.1 to 10.0 h−1.
The process of the present invention presents considerable differences as regards objectives, operating conditions and results compared with those of the above references, since it is effected by a combination of low-pressure operating conditions, of the type of reactor and of the type of feedstock to be hydrotreated, which together provide a high capacity for removal of metals, sulfur, nitrogen and asphaltenes, as well as limiting the formation of sediments and sludge, to obtain a hydrotreated hydrocarbon of improved properties; which are presented with clarity and detail in the following chapters.